1. Field of the Invention
The present invention relates to an automatic impedance adjuster for adjusting an impedance of a probe head for transmitting/receiving a radio-frequence (RF) signal to be equal to a characteristic impedance of a feeder line in a magnetic resonance imaging (MRI) system.
2. Description of the Related Art
A probe head in the MRI system serves as an antenna for transmitting an RF signal for exciting a magnetic resonance (RF) phenomenon, and for receiving an RF signal, i.e., an MR signal, generated by the MR pnenomenon. The probe head includes one type commonly used for both transmission and reception of an RF signal, and the other type used for only reception of an RF signal. The probe head has an RF coil consisting of saddle coils.
FIG. 1 shows an equivalent circuit of a probe head. The probe head is connected to a cable of characteristic impedance Zc (in many cases, Zc=50 .OMEGA.). A probe head of a type used for both transmission and reception is usually fixed at a specific position. However, a reception type probe head is arranged at a variety of positions in accordance with diagnosis modes. The probe head represented by this equivalent circuit is constituted by variable capacitors C.sub.1V, C.sub.2A, and C.sub.2B (the capacitance of variable capacitor C.sub.1V is C.sub.1, and capacitances of variable capacitors C.sub.2A and C.sub.2B are respectively C.sub.2) connected to an RF coil, and equivalent inductance L of the RF coil, and equivalent parallel resistor R of the RF coil. Note that reference symbol Zi denotes an input impedance of the probe head.
When this probe head is used, input impedance Zi must be adjusted to be equal to characteristic impedance Zc of a cable.
This is for the following two reasons. (1) If Zi.noteq.Zc, transmission loss of an MR signal occurs in a feeder line, and S/N characteristics are degraded. (2) A low-noise amplifier used in an MRI system is normally designed to have input impedance Zc in correspondence with a cable normally used. Therefore, if Zi.noteq.Zc, i.e., if the amplifier is connected to a signal source having a signal source impedance other than Zc, a low-noise component cannot be assured since noise figure matching is impaired.
In the equivalent circuit of FIG. 1, if both resistance R and impedance Zi are real numbers, the relationships between capacitances C.sub.1 and C.sub.2 and impedance Zi are respectively represented as follows. ##EQU1## (.omega.=2.pi.f; f [Hz] is a resonance frequency of an MR signal)
As can be seen from equations (1) and (2), if impedance Zi (which is set to be a pure resistance) is increased, capacitance C.sub.2 is decreased, and capacitance C.sub.1 is increased. More specifically, if resistance R and inductance L are constant, capacitance C.sub.2 can be decreased (capacitance C.sub.1 is slightly increased accordingly), so that impedance Zi as a pure resistance can be increased.
From equations (1) and (2), impedance Z.sub.X of a circuit portion surrounded by a broken line in FIG. 1 is calculated as follows: ##EQU2## In equation (3), the first term, i.e., Zi represents a pure resistance component, and the second term, i.e., j.sqroot.Zi(R-Zi) represents an inductive reactance.
Impedance Z.sub.Y of a circuit portion constituted by two capacitors C.sub.2A and C.sub.2B can be represented by: ##EQU3##
As can be seen from equations (3) and (4), impedance Zi is constituted by impedance Z.sub.X formed by capacitance C.sub.1, resistance R, and inductance L, and impedance Z.sub.Y formed by two capacitances C.sub.2. In order to obtain impedance Zi as a pure resistance, an imaginary part (inductive reactance component) in equation (3) can be canceled to zero by an imaginary part (capacitive reactance component) in equation (4).
As described above, in order to satisfy condition Zi=Zc, if Zi&gt;Zc, adjustment is performed as follows. That is, capacitance C.sub.2 is decreased, and capacitance C.sub.1 is increased to cancel the imaginary part generated when capacitance C.sub.2 is decreased, so that impedance Zi is decreased. If Zi&lt;Zc, adjustment is performed as follows. Capacitance C.sub.2 is increased and capacitance C.sub.1 is increased, so that impedance Zi is decreased.
The probe head itself is required to have high Q (quality factor). Since an object to be examined (human body) comes closer to the probe head during imaging, the equivalent circuit as a primary approximation of the probe head during actual imaging is as shown in FIG. 2. Capacitances Cs and Cs' are stray capacitances between an object and a coil, and resistance Rp is an equivalent resistance of the object. Therefore, since these capacitances Cs and Cs' and resistance Rp are added, an impedance is not Zi but Zi'. The arrangement of FIG. 2 can be equivalently transformed, as shown in FIG. 3, and the arrangement of FIG. 3 can be further simplified, as shown in FIG. 4. Capacitance Cs" and resistance Rp' shown in FIG. 3 are respectively an equivalent capacitance and an equivalent resistance when a circuit consisting of capacitances Cs and Cs' and resistance Rp is parallel-transformed. In FIG. 4, ##EQU4## In the equivalent circuit of FIG. 4, the relationships between capacitances C.sub.2 of variable capacitors C.sub.2A and C.sub.2B, capacitance C.sub.1 ' of variable capacitor C.sub.1V and impedance Zc are represented by the following equations based on condition Zi'=Zc in substantially the same manner as in equations (1) and (2): ##EQU5##
Therefore, when capacitances C.sub.1 ' and C.sub.2 are adjusted in the same manner as in FIG. 1, condition Zi=Zc can be satisfied.
In a conventional apparatus, this adjustment is performed as follows. An oscillator having output impedance Zc (pure resistance) is prepared. A directional coupler is inserted between the oscillator and the probe head. A reflection power of the oscillator output by the probe head is derived and monitored through the directional coupler, and capacitances C.sub.1 ' and C.sub.2 are varied, so that a condition yielding reflection power=0 is detected in a trial and error manner.
In this manner, in the conventional adjusting method, two variable capacitors C.sub.1 ' and C.sub.2 must be simultaneously adjusted by manual operation, resulting in a cumbersome adjustment. In this case, the adjustment often depends on skills or experiences, resulting in poor work efficiency.
Instead of the manual operation, an automatic adjusting method using a microprocessor and the like is proposed. However, most adjustment is performed depending on the decision function of the microprocessor, and this also results in poor adjustment efficiency.